Yes, there is a relationship between weight and sail size. However, it is also affected by wind speed. (Stronger wind = smaller sail.) I made a spreadsheet that will calculate the correct sail size for your weight and the wind speed. Check it out:

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But would like to add (from my experience and not inconsierable weight (103kg) that higher winds reduce difference in sail size required.(more than you would expect)
For example when 80k mates are on 6 metres I`ll be on 6.5 (or perhaps 7) but by time it gets to 4 metre weather for them I can also get going on same size.
Other thing I`ve picked up is that heavier sailor can push sails above point where they are stable.Sounds obvious but can make sail choice a real problem for heavies.For example the NP Expression is damned good sail and has quite a good range but (IMO) is a poor choice for heavies because its top end is poor.(Even though it has good bottom end power)
Heavier sailors actually need sails with excellent top end control (for size)
Dont think lighter sailors can load sails up enough (relatively) to point where they "break up".They would just feel overpowered;which is different to sail "breaking up".Expression might have a fantastic range for 70k rider but not for a 100.On other hand the Tush Storm has great range for both.(But doesn`t have bottom end of Exp)